1. Field of the Invention
This invention is related to a film scroll detection mechanism of a camera.
2. Description of Related Art
FIG. 4 shows a cross-sectional view of a conventional film scroll detection mechanism of a camera. This conventional film scroll detection mechanism of a camera contains a film scroll detection roller (31), which moves in conjunction with the scrolling of the film (14), and a pulse signal generation mechanism for converting movement of the film scroll detection roller (31) into a pulse signal.
Detection of scrolling of the film is achieved by means of an encoder (33) containing transparent parts (53) and opaque parts (54) provided at certain fixed angles as shown in FIG. 3B. The encoder is placed on the same shaft as the main axis of rotation (32) of the film scroll detection roller (31), and this is supported by the main body (36) and a mounting part (35), and combined with a photo-interrupter (34). A spring 38 keeps the film (14) in contact with the film scroll detection roller (31). Item (39) is the back lid of the camera and item (37) is the back cover of the camera. Item (40) is a pressure plate that maintains film (14) in position, as is well known.
When movement of the film scroll detection roller (31) is converted into a pulse signal that is used not only for detection of the movement of one frame of the film but also, for example, as a pulse signal for the mechanism that displays the date concurrently with the scrolling of the film, a very large number of signals are required (e.g., 180 pulses per frame).
In this instance, in using the photo-interrupter to produce the pulse signal, it is necessary to have an encoder containing numerous transparent and opaque parts, each having a specified width. The reason for this is that when the width of these parts is less than the specified width, it is difficult to pass light reliably through the transparent parts, making it impossible for the photo-interrupter to perform its function accurately.
In conventional devices that use an encoder (33) that rotates the same amount as the film scroll detection roller (31), it is necessary to have a large number of transparent parts (53) and opaque parts (54) on the encoder in order to produce a requisite number of pulses.
In addition, it is necessary to have a certain width (for instance, 0.1 mm or 0.3 mm) for these transparent and opaque parts, making it necessary to enlarge the radius (f) of the encoder (33).
Furthermore, because the film scroll detection roller (31) is kept in contact with the film (14), the distance (e) from the surface of the film to the axis of the roller (31) is predetermined. As a result, when an encoder (33) with a large radius (f) is mounted directly to the roller (31) so that it shares the same axis of rotation as the film scroll detection roller, the distance (g) from the surface of the film to the back of the camera becomes large. These restrictions create a large obstacle to making the camera smaller.